In underground mines, a main vertical shaft generally provides vertical access to all working levels. Oftentimes, exploratory drilling indicates that additional ore lies beneath the deepest level which is below the access provided by the shaft.
In these instances, it is necessary to provide access to these additional reserves by either deepening the existing shaft or developing a decline system.
As can be readily appreciated, deepening an existing shaft is a difficult undertaking at best. Besides being disruptive to the normal operation of the mine, deepening the shaft is time consuming, expensive and fraught with safety considerations.
Large diameter shafts are being drilled in the United States and elsewhere. While not widely practiced, several techniques are used.
Most of these shafts are drilled from the surface using modified oil rigs. Double or triple wall drill string is often used to permit two-way travel of bailing fluid to and from the cutting face. Multi-phase systems employing injected air to assist circulation have been used with varying degrees of success. Reverse circulation systems are the most widely used, with bentonite mud or water as the preferred media.
Fluid jets are used to agitate the cuttings as they are created and to clean the rock area ahead of the cutter prior to contact. The suspended cuttings swirl with the rotation of the cutterhead, spiralling towards a central pickup point for hydraulic transport through the string to surface. The cuttings are removed through a series of cyclones, screens and desilters prior to recirculation of the hydraulic fluid.
The vast majority of these shafts are drilled in the softer sediments associated with coal deposits, with stratified lithologies and water-bearing horizons. It is advantageous in some cases to maintain a high fluid level in the hole during drilling, which provides hydrostatic support to the shaft walls. After drilling is complete, shaft liners can be floated into place and pinned or grouted.
The Sudbury, Ontario, Canada rock formations are much harder and different equipment is necessary to provide satisfactory drilling performance. Carbide cutters must be used to provide reasonable penetration rates and cutter life. A much greater proportion of fines are produced during drilling which affect the design and selection of a bailing system.
The size and power requirements of the drill rig, and the costs associated with the multiple wall drill string and fluid cleaning equipment generally preclude it from consideration as a feasible means of drilling shafts in an underground hard rock environment.